1. Field of the Invention
The present invention relates generally to an electrostatic capacity-type stroke sensor for monitoring displacement stroke of a movable member. The invention also relates to a dielectric material suitable for use in electrostatic capacity-type sensors. Further, the invention relates to a stroke sensor suitable for monitoring relative displacement between a sprung mass and an unsprung mass in a vehicular suspension system.
2. Description of the Background Art
Electrostatic capacity-type sensors have been widely used for monitoring displacement of a movable member. For example, the electrostatic capacity-type stroke sensor has been employed for monitoring relative displacement between a vehicle body as a sprung mass and a vehicular suspension member as an unsprung mass. Such an electrostatic capacity-type stroke sensor has been disclosed in the Japanese Utility Model First (unexamined) Publication (Jikkai) Showa 60-105422. In the disclosed construction, the electrostatic capacity-type stroke sensor includes a cylindrical casing and a rod member displaceable relative to the cylindrical casing. The cylindrical casing and rod member are so arranged as to establish variable electrostatic capacity depending upon the relative position of the rod member and cylindrical casing. Namely, in the shown construction, the rod member and the cylindrical casing are made of an electrically conductive material. The conductive rod member and cylindrical casing are electrically isolated from each other. On the other hand, an electrically conductive cylindrical casing is arranged coaxially with the cylindrical casing and fitted onto the outer periphery of the cylindrical casing. This conductive casing serves as an opposing electrode for the cylindrical casing. The cylindrical casing is connected to a stroke detector circuit.
The rod member is electrically connected to the conductive casing via a brush slidingly in contact with the outer periphery of the rod member. The conductive casing is, in turn connected to the stroke detector circuit. With this construction, electrostatic capacity is established between the rod member and the cylindrical casing. Since this electrostatic capacity is variable depending upon the relative position of the rod member and the cylindrical casing according to a predetermined relationship therebetween, the magnitude of relative displacement of the rod member and the cylindrical casing can be derived by measuring the electrostatic capacity created therebetween.
Though such an electrostatic capacity-type stroke sensor allows precise measurement of the relative stroke of the rod member and the cylindrical casing and allows precise measurement of the relative position of the vehicle body and the suspension member, a drawback may be encountered with respect to durability. Such a drawback is created by use of a contact type electrical connection between the rod member and brush. Namely, as is well known, the rod member frequently strokes in bounding and rebounding directions when absorbing vehicle body motion and road shock input through vehicular wheels. Therefore, the rod member frequently slides relative to the brush to cause wearing or abrasion of the brush. Furthermore, a sliding resistance created between the rod member and the brush may cause variation of the electrostatic capacity to degrade precision of the measured stroke.
In addition, such a construction requires a brush in sliding contact with the rod member and a lubrication mechanism for providing satisfactory lubrication between the brush and the rod member. This makes the construction of the sensor too complicated.